This invention relates to a method of x-ray imaging with apparatus having a source of ionizing radiation, and an imaging device for converting the radiation to a wavelength suitable for detection by a television camera. More specifically, this invention relates to a method of x-ray slit scanning with controlled TV camera tube target erase so as to reduce substantially the effects of scatter.
A typical use for such imaging is as a medical diagnostic modality. In such applications, primary imaging radiation, such as x-ray radiation which has been intensity modulated by passage through a patient, strikes the input screen of an image-intensifier tube where it is converted to an electron latent image. Electrodes contained in the tube minify the image and accelerate the electrons toward a luminescent output screen of the image-intensifier tube. An image having increased brightness is produced on the output screen in accordance with the spatial modulation on the x-ray radiation. A television camera and monitor are used to display the image. Frequently, a photographic camera is also used to record images of diagnostic interest.
High image quality, as measured by image resolution, contrast, and x-ray photon noise, is very desirable in such applications of the imaging apparatus. Image quality is, however, degraded by scatter which affects contrast and noise. One type of scatter, which will be referred to as x-ray scatter, is produced by deflection of x-rays from their normal paths by the body undergoing examination. Such x-rays strike the input screen of the image-intensifier tube and induce a spurious response which is detected by the television camera, for example, and displayed on the television monitor. Additional scatter is produced by scattered electrons in the image-intensifier tube and by other system light optical elements, further degrading image quality. Scatter acts to increase the overall brightness of the image background thereby decreasing image contrast. Therefore, it will be appreciated that minimizing the effects of scatter will have the desirable effect of increasing image contrast, reducing noise, and concomitantly improving image quality, since both scatter and primary x-ray radiation contribute to x-ray photon noise.
One known method of reducing the deleterious effects of x-ray scatter is to position a radiation-absorbing grid in front of the input surface of an imaging device, such as an image-intensifier tube, a fluoroscopic phosphor screen, or a photographic camera. An example of an imaging apparatus using a grid is disclosed in U.S. Pat. No. 4,220,890. Undesirable effects associated with the use of such a grid include the attenuation of non-scattered primary imaging radiation producing grid shadow lines which can obscure image detail behind the shadow. Attenuation of the primary imaging radiation also has the effect of decreasing the signal-to-noise ratio of the image. Additionally, radiation-absorbing grids do not reduce scatter originating in the image-intensifier tube and in system optical elements.
Another method of scatter reduction is described by G. T. Barnes and I. A. Brezovich, "The Design and Performance of a Scanning Multiple Slit Assembly, " Medical Physics, Vol. 6, No. 3 (May/June 1979), pp. 197-204. In this method, a series of narrow slits for collimating the x-ray beam is positioned between the x-ray source and the patient. A second series of scatter-eliminating slits is disposed between the patient and the x-ray conversion device (e.g., an x-ray image-intensifier tube) and are aligned with the x-ray collimating slits. During an exposure, the slits are synchronously moved to scan the object in a direction perpendicular to the slits such that both sets of slits and the x-ray source focal spot are maintained in the same plane. In this manner, the scattered radiation is prevented from reaching the x-ray conversion device. A drawback associated with this method is the difficulty in maintaining positional slit synchronism. Additionally, such apparatus is subject to reliability problems commonly encountered with moving parts of a mechanism.